In the rotary drilling of wells, a drilling circulation fluid is circulated down the drill string, past the bit and back up to the surface in the annulus between the drill string and the borehole wall. The typical drilling fluid, or mud, contains water and/or oil, finely divided solids, and various additives which impart the desired gel strength, viscosity, weight and filtrate loss properties to the mud.
Typically, after the well has been drilled to a desired depth, a permanent casing is cemented in place. Cementing is accomplished by pumping a cementitious slurry into the annulus between the casing and the borehole wall where it is allowed to set, thus binding the casing to the formation. For the purposes of this invention, the term "casing" will apply to both casing and liner.
Obtaining a complete displacement of the circulating fluid in place in the annulus by the cement slurry is a primary consideration in cementing operations. Inadequate displacement of the mud leaves channels of gelled mud remaining in the annulus after the cement is in place which greatly reduces the effectiveness of the cementing operation. Moreover, the mud may contain components which prevent or delay the setting of the cement or reduce the strength of the set cement.
Removing the soft filter cake deposited by the mud on the borehole wall during the drilling operation is an equally important consideration. A soft filter cake presents an inadequate surface for bonding of the cement and is perhaps the most common cause of primary cementing failures. As this soft filter cake increases in thickness, the likelihood that the primary cementing job will fail also increases.
Because the drill bit must necessarily be as large or larger than the outside diameter of the casing, the drill string and bit must be withdrawn before the casing can be cemented in the borehole. The "tripping" of the drill string and suspending the casing takes considerable time, as much as twenty-four hours for deep wells. During this time, the drilling mud in the borehole cannot be circulated, and its gel strength has a tendency to increase when not in circulation, especially when subjected to high temperatures which may degrade the dispersants and thinners therein, requiring a high rate of shear to overcome the gel strength and re-initiate flow. Moreover, filtrate loss into the permeable formation results in buildup of filter cake during the trip of the drill string. Thus, the delay in cementing often results in ineffective displacement of the mud by the cement due to the increased gel strength and an ineffective bond between the cement and the borehole wall because of the filter cake that has been deposited.
There have been many attempts to eliminate the channeling of the mud as it is displaced by the cement slurry and to reduce the thickness of the deposited filter cake so that the bond of the cement to the borehole wall is improved. The mud is usually conditioned before tripping the drill string to ensure that drill cuttings are removed and the viscosity and gel strength are reduced. However, the mud still contains finely divided solids which cause filter cake to build up during the tripping of the drill string. These finely divided particles and the resultant filter cake have heretofore been thought to be essential to prevent the loss of the borehole fluids while tripping the drill string and suspending the casing.
One attempt to remove the filter cake involves the use of scratchers attached to the outside of the casing. The scratching appliance is a ring with flexible steel wires attached to it. As the casing is lowered into the borehole, it is reciprocated or rotated, and the scratchers stir the mud in place in the borehole and scratch off some of the filter cake. However, scratchers only agitate the mud in their localized vicinity. Moreover, movement of the casing must be maintained until the cement enters the annulus or the filter cake will reform.
Attempts to eliminate mud channeling include flushing the mud from the annulus after setting the casing, such as according to the method described in U.S. Pat. No. 3,884,302. While flushing may eliminate mud channeling in the cement, it has little effect in preventing formation of or removal of filter cake. There is described in U.S. Pat. No. 4,302,341 an effort to provide a preflushing or spacer fluid which aids in removing the filter cake by adding relatively large, angular solids to the preflushing fluid and flowing the fluid through the annulus at a high, turbulent flow rate. However, a high rate of flow results in an excessive downhole pressure which may fracture the formation, an event which requires time-consuming and expensive remedial operations. Moreover, reducing the flow rate or interrupting the flow will permit the filter cake to reform.
Another method for casing cementation is described in U.S. Pat. No. 3,653,441 in which a liquid slug having a graded viscosity is injected into the annulus ahead of the cement slurry. The liquid slug has a leading edge having a viscosity greater than the viscosity of the drilling mud and a trailing edge having a viscosity less than the viscosity of the cement slurry. As with the preflushing methods, the use of this graded viscosity slug does not provide for preventing filter cake buildup during removal of the drill string and suspension of the casing because the liquid slug contains undissolved solids, e.g. carbonate powder, used to control the viscosity.
In contrast to the methods heretofore known, the present invention provides a method of cementing a casing into a borehole in which there is no buildup of filter cake during removal of the drill string and suspension of the casing. The method of the present invention thus improves the bonding between the set cement and the formation.